I updated to Newsletter 3.1.3 today and now I cannot preview new posts. If I deactivate Newsletter, I can preview, but not with Newsletter activated. When I try to preview, I can see the title, but the content of the post is missing. The content appears when I deactivate Newsletter.
Hi the double slash usually is not a problem. Anyway Newsletter get the images url from wordpress, so looking at the source generated by newsletter and copying an image url to the browser an error should be displayed to better understand where the problem is.
May be in your server the double slash is a problem.